The invention relates to a method for manufacturing flat-bottom bags with a glued-in bottom lining.
In the case of flat-bottom bags, such as block flat-bottom bags or cross flat-bottom bags of paper, plastic film or the like, the bottom flaps form, on the inside of the bag, undesirable hollow spaces, such as gaps or pockets, in which powdery, grainy or pasty residues of the contents of the bag can collect. It is therefore known that a so-called bottom lining may be glued into the bottom of the bag on the inside. Usually, this bottom lining is formed by a rectangular sheet of the bag material and is glued or welded all around to the bottom flap, so that the hollow spaces are closed off from the interior of the bag.
The WO 95/02544 discloses a method for manufacturing such bags, in which the bag material, as well as the bottom lining, consists of paper coated on the inside with polyethylene. According to this method, the bottom lining is glued at the edges, which run parallel to the narrow edges of the bag, to the associated bottom flap and, parallel to the edges at the broad side of the bottom, the mutually facing coated surfaces of the bottom lining and the bottom flap are welded together by means of continuous weld lines.
However, the gluing or welding of the bottom lining generally is relatively difficult and expensive technologically because there is no direct access to the inside of the bottom for the tools required. The weldings are therefore generally carried out with the bottom placed flat, so that there are several layers of material of the bottom between the welding tools. Generally, therefore, by suitably controlling the process or by inserting separator sheets or the like, it is ensured that the welding takes place selectively only at the desired places and that material layers, which must be separated from one another once again later on when the bag is being filled, are not welded together. Similar problems also arise with gluings, because the adhesive, usually glue or a hot melt adhesive, must be applied in such a manner during the manufacturing process, that undesired glued sites do not result and that the bottom flaps are not smudged with adhesive when the bottom lining is put in place.
The usual method for manufacturing flat-bottom bags consists therein that the starting material, in the form of an endless sheet, is pulled off from a roll and then, initially, closed into a tube, which is then divided into sections of suitable length for forming the individual bags. The pieces of tube, so obtained, are supplied continuously to a rotating bottom-laying drum, at which an opening station, a bottom-laying station and a closing station are disposed consecutively in the circumferential direction. In the opening station, the upper and lower layers of the tube, which initially lie flat on top of one another, are pulled apart at the bottom end, so that a wide open bottom opening results. In the bottom-laying station, the edges of this bottom opening, opposite to one another in the running direction, are pulled apart and the longitudinal edges are folded over, so that bottom flaps are formed, which partially overlap one another and lie flat against the periphery of the bottom-laying drum. The outwardly-folded flaps are then folded over in the closing station and, by means of a final gluing, glued together so that a closed bottom is obtained.
If the bottom is to be provided with a bottom lining, then this lining must be introduced into the bottom opening between the opening station and the bottom-laying station and then welded to or glued to the bottom flaps. Due to the gluing and/or welding facilities required for this purpose, the construction of the apparatus is made considerably more complicated and, since the additional steps of the process, such as the application of the glue, the production of the weld lines and the like, require a certain amount of time, the total processing time is increased so that only a correspondingly lower output of bags can be achieved in the continuous manufacturing process.